Roles are composable units of behavior. They are useful for factoring out
functionality common to many classes from any part of your class hierarchy.
(See Moose::Cookbook::Roles::Recipe1 for an introduction to Moose::Role.)
While combining roles affords you a great deal of flexibility, individual
roles have very little in the way of configurability. Core Moose provides
alias for renaming methods to avoid conflicts, and excludes for ignoring
methods you don't want or need (see Moose::Cookbook::Roles::Recipe2 for more
about alias and excludes).
Because roles serve many different masters, they usually provide only the
least common denominator of functionality. To empower roles further, more
configurability than alias and excludes is required. Perhaps your role needs
to know which method to call when it is done. Or what default value to use for
its url attribute.
Parameterized roles offer exactly this solution.